Diversity techniques are widely used in wireless communications to improve the signal performance. Spatial diversity typically uses two or more antennas spatially separated. The system performance is generally limited by the cross-correlation coefficient between the two spatially diversified antennas. The optimum performance occurs only when the cross-correlation coefficient approaches zero.
Polarization diversity provides an alternative to spatial diversity for base station communications. It has been widely used in cellular system (GSM), Personal Communications Services (PCS), and more recent systems including advanced wireless service (AWS) and WiMax. In actual communication systems, signals encounter multi-path propagation and multiple reflections, which cause depolarization of the signal. As a result, the antenna at the base station need not be aligned with vertical linear polarization. A dual polarization base station antenna creates two branches by using an antenna with dual simultaneous polarizations oriented orthogonally to each other. In general, the two branches of the dual polarization base station antennas are implemented by planar radiating elements slanted at +45° and −45° with respect to the main axis of the antenna. For each branch, the slant 45° polarized antenna signals may be represented by two polarization components, one polarization component that is vertical and the other that is horizontal, namely E(theta) and E(phi), respectively. The slanting angle (θ) of the radiating element depends on the E(theta) and E(phi),
                    θ        =                              ±            tan                    ⁢                                          ⁢                                    E              phi                                      E              theta                                                          (        1        )            
Theoretically, E(theta) and E(phi) need to maintain the same power across an azimuth (typically horizontal) cut in order to provide an effective performance for a wireless base station site with perfectly slanted ±45° dual polarized fields over the field of view of the antenna. The ideal antenna transmission pattern has substantially rotationally symmetric E(theta) and E(phi) radiation patterns along a typical 120° sector coverage with E(theta) equal to E(phi) over the entire range. In reality, prior art base station antennas are not able to maintain E(theta) and E(phi) within 3 dB of each other over an azimuth range equal to 120° sector coverage. In addition, an ideal antenna performance would maintain E(theta) and E(phi) equal to each other over the entire applicable bandwidth, which typically covers a 30% range from about from 85% to about 115% of the center frequency. Again, in reality, prior art base station antennas are not able to maintain E(theta) and E(phi) within 3 dB of each other over a 30% bandwidth as well as over a 120° azimuth range.
In the wireless communication industry, base station antennas with different azimuth beamwidths are required by many operators. The azimuth beamwidths range between 18° and 120°. With a dipole, the azimuth beamwidth can be easily achieved below 65°. Multiple columns of dipoles with predetermined power distribution can achieve azimuth beamwidths as low as 18° with both E(theta) and E(phi) exhibiting similar signal strength along the 3 dB beamwidth coverage. For azimuth beamwidth above 65°, however, the E(phi) 3 dB beamwidth is limited to around 70° due to the nature of the dipole. In order to achieve wider azimuth beamwidth, the prior solution has been to increase the beamwidth of the E(theta). With this technique, the antenna loses the rotationally symmetric radiation patterns, and the slant 45° dipole leans to a smaller slanting angle based on the differences between E(theta) and E(phi) as described by equation (1), which causes the dual polarized dipoles to no longer be orthogonal to each other. As a result, the communication performance drops near the edge of the cell, potentially causing more dropped calls between the mobile unit and the base station.
Runyon, U.S. Pat. No. 6,067,053, describes a dipole antenna element with drooped dipole arms that can improve the radiation pattern performance by increasing the E(phi) 3 dB beamwidth to more than 70°. However, the matching bandwidth of the droop arm dipole is limited by its nature to less than 10% to fit the PCS frequency band from 1850-1990MHz, equivalent to seven percent of the center frequency. As a result, the beam pattern performance is limited to the PCS frequency band for which the dipole was designed. In general, prior art dual polarization base station antennas have not been able to achieve beam pattern performance with the E(theta) and E(phi) 3 dB beamwidths within 5° of each other, while maintaining E(theta) and E(phi) within 3 dB of each other over a wide beamwidth, such as 120°, and over a wide bandwidth, such as 30% of the center frequency.
As a result, there is an ongoing need for dual polarization base station antennas with improved E(theta) and E(phi) beam pattern performance characteristics. In particular, there is an ongoing need for dual polarization base station antennas that can achieve beam pattern performance with the E(theta) and E(phi) 3 dB beamwidths within 5° of each other, while maintaining E(theta) and E(phi) within 3 dB of each other over a wide beamwidth, such as 120°, and over a wide bandwidth, such as 30% of the center frequency